1. Field of the Invention
The present invention relates to new and useful corrosion inhibitors for reducing the acid attack on or corrosion rate of, a ferrous metal surface by acidic chemical cleaning solutions. More particularly, the present invention concerns employing a novel inhibitor composition in an aqueous acidic hydrofluoric acid cleaning solution to materially reduce the acid attack on ferrous metal surfaces and particularly on surfaces of highly acid-reactive ferrous metals.
2. Description of the Prior Art
Metal surfaces have been cleaned of oxide scale incrustations by employing aqueous solutions of strong and weak acids, in which there is often employed an inhibitor to reduce the attack of the aqueous acidic solution on the metal surface. For example, mineral acids such as sulfuric, sulfamic, hydrochloric and phosphoric acids have been employed for scale removal and metal cleaning. Frequently organic acids such as formic, acetic, citric, propionic, and the like have been employed also for metal cleaning. When such acid cleaning solutions have been employed for the removal of oxide scale and mineral incrustation on a ferrous metal surface, the metal surface is also attacked.
To reduce the attack of acid solutions on the metal surface, it has been known, for example from U.S. Pat. No. 3,077,454 to employ a reaction product of an active hydrogen-containing compound, an alpha ketone, an aldehyde and a fatty acid.
The active hydrogen compound employed therein is an organic ammonia derivative having at least one hydrogen atom attached to a nitrogen. Such active hydrogen compounds thus include, inter alia, primary and secondary amines, diamines, amides, ureas, thioureas, ammonia, ammonium salts, alicyclic amines and the like.
From U.S. Pat. No. 3,634,270 it is known to employ the reaction product of U.S. Pat. No. 3,077,454 together with a sulfur compound to prevent the corrosion of metals in contact with corrosive materials such as HCl, H.sub.2 SO.sub.4 or alkylene diamine tetracetic acid partially ammoniated salt. The sulfur compound can be thiourea, its aliphatic homologs, sulfides, thiocyanates, cyanates and the like.
It is known from U.S. Pat. No. 2,959,555 to employ a urea derivative in a hydrochloric, sulfuric or phosphonic acid solution to remove copper containing iron oxide incrustation from an iron surface. Such a solution can also employ an organic nitrogen-base type of corrosion inhibitor.
Mannich bases and particularly rosin amine Mannich base reaction products have been found useful for inhibiting the attack of a metal surface by acid solutions. For example, from U.S. Pat. No. 2,758,970 it is known that a rosin amine -- formaldehyde -- ketone reaction product is useful for the protection of ferrous metals during the removal of water-borne deposits, of heat scale and of rust where removal is carried out by means of acid salts or acids such as hydrochloric, sulfuric, acetic, formic and the like.
Various other nitrogen-containing inhibitor bases have been employed to reduce the attack on metal surfaces by acidic solutions. For example it is known from U.S. Pat. No. 3,668,137 to employ such an "inhibitor base" together with a carboxylic acid and a sulfonium salt to inhibit acid attack of a metallic surface. The "inhibitor base" can be a nitrogen-containing compound such as the coal tar bases (pyridine and its derivatives) and their quaternary salts, alkyl amines, aryl amines, ethylene oxide condensates of amines, and coal tar Mannich bases which in themselves have acid attack inhibiting properties. The "inhibitor base" alternatively can be a nitrogen-containing compound such as an amide, urea, thiourea, acetamide, formamide, and 1-phenyl-2-thiourea.
Acid cleaning solutions employing acid inhibitors of the types described above have been quite satisfactory for many purposes. Recently, with the advent of nuclear fuels coupled with increased demand for more power, now construction materials have been required for boiler walls, super heaters, and the like. To efficiently accomodate the higher temperatures required to produce more power with greater efficiency, steel alloys comprising chromium, copper, and magnesium have been employed. These steel alloys also frequently contain nickel, molybdenum, aluminum, and columbium. When it has been desired to clean such steel alloys of deposited scale and metal oxides, aqueous acidic solutions as were used on the more common alloys, comprising about 5% of hydrochloric acid have been employed. Of course such cleaning solutions contained inhibiting compositions to reduce the metal dissolution rate of the metal surface in contact with the cleaning solution. The steel alloys from which some boiler walls, super heaters and the like are constructed are so highly reactive with the acidic cleaning solutions that excessive losses of both acid and steel alloy were experienced and yet cleaning required from 24 to 48 hours.
It was then found in the art that hydrofluoric acid was a more efficient solvent of scale and metal oxides. A satisfactory acidic cleaning solution can employ hydrofluoric acid in the amount of about 1 or 2% by weight. Moreover, the cleaning time could be cut in half over that required by the hydrochloric acid solutions.
However, most known inhibiting compositions have been found to be unsuitable for inhibiting the acid attack of an aqueous cleaning solution based on hydrofluoric acid. Many known inhibitors, or inhibiting compositions either decompose in a hydrofluoric acid cleaning solution or interact at the metal surface in contact with the inhibited cleaning solution, causing a darkened surface appearance. Most known inhibitors, when used to protect a ferrous metal surface during an aqueous hydrofluoric acid cleaning operation have been found to be totally inadequate for protecting the highly reactive steel alloys, and in fact, frequently completely lose their protective influence above about 50.degree. to 60.degree. C. Generally the known inhibiting compositions fail to provide sufficient protection against acid attack by hydrofluoric acid cleaning solutions in contact with the highly reactive metal surfaces particularly when it is desired to inhibit the acid attack such that metal dissolution is less than about 1.01 g/m.sup.2 /hr.